An important area of research in the field of medical devices is the development of a prosthetic nucleus for a vertebral disc. In U.S. Pat. No. 5,047,055, a prosthetic nucleus for implanting in the disc space after the removal of a degenerated or damaged nucleus of an intervertebral disc is disclosed and claimed, as well as a method for forming the prosthetic nucleus. That patent is hereby incorporated herein by reference.
In making such a prosthetic nucleus for insertion, the implants made of bulky hydrogels should be dehydrated so that they have a water content as low as possible. This allows for easy insertion of the disc component during surgery due to the accompanying reduction in volume. Additionally, it is desirable for the implant to have no distortions (i.e., no concavities or dimplings formed therein) and it is desirable that the apparent volume (defined herein to be the volume that the disc would have if it had no concavities and is equal to the largest cross-section of the disc multiplied by the largest disc width) of the implant be a minimum. Furthermore, it is desirable that the implant have no sharp edges. It is especially important to have a minimum dehydrated volume for an implant which is to be inserted percutaneously.
However, it has been found that when the bulky hydrogels as disclosed in U.S. Pat. No. 5,047,055 are dehydrated so as to reduce the water content as low as possible, under certain conditions, gross distortions of the implants occur. These distortions may not be acceptable in the intended medical application. For example, sharp edges on the superior and inferior surfaces of the implants can cause damage on the end-plates of the natural disc.
It is an object of this invention to produce vertebral disc implants which have a water content as low as possible, which exhibit (to the naked eye) no visible distortion of the implants and no sharp edges, and which have a minimum apparent volume.
Another object of this invention is a method of dehydration of a bulky hydrogel implant so as to maintain the original shape of the hydrated implant.
Yet another object of this invention is an implant having a water content which is as low as possible (i.e., below about 10%) and which has a minimal apparent volume, no sharp edges, and no visible distortion (that is, no visible concavities) in the implant.
Yet another object of this invention is to dehydrate vertebral disc implants to a consistent geometry and size.